1. Field of the Invention
This invention relates to an electromagnet apparatus used, for example, for making and breaking a contact for an electromagnetic switch, and more particularly to a power supply circuit for such an electromagnet apparatus.
2. Description of the Prior Art
FIGS. 1 and 2 show a prior art electromagnet apparatus as disclosed in Japanese Utility Model Laid-Open No. 84-59448, wherein a full-wave rectifier 1 converts a.c. current into d.c. current supplied to a coil 2. A holding current capacitor 3 in series with a resistor 4 is connected in parallel across a closure switch 5. The resistor 4 suppresses the large transient discharge current which would otherwise flow through the switch 5 upon its initial closure due to the residual charge build up on the capacitor 3.
The coil 2 is wound around a fixed iron core 6 associated with a movable iron armature 7, which is magnetically attracted to the fixed core when the coil is energized. The switch 5 is closed when it is desired to attract the movable armature 7 to the fixed core 6 from a released state shown in FIG. 2; it is opened shortly before the armature reaches the fixed core, whereafter a reduced amplitude current flows through the capacitor 3 and resistor 4 to continue the attractive movement of the armature until it engages the fixed core 6 and then holds the armature in such engagement.
The movable armature 7 is thus attracted by a large magnetic force when the gap is large, and held by a smaller magnetic force after being attracted. The holding power consumed by the coil is therefore reduced, and since only d.c. current flows through the coil no electromagnetic noise or chatter occurs.
With such an arrangement, if the applied a.c. voltage is interrupted to release the armature, a flywheel current flow arises due to inductive energy stored in the coil 2 as shown by the arrow If in FIG. 1. Such current is progressively damped by the L-R circuit of the coil, and the movable armature 7 is released when the attractive force of the electromagnet becomes lower than the repulsive force of a tripping spring (not shown) This flywheel effect extends the release period of the movable armature, and the delay time increases in proportion to the inductance value of the electromagnet coil 2. Such release delay renders the prior art circuit unsuitable for making and breaking switch contacts in situations in which precise timing control is essential.